The binding of pertussis toxin and it B oligomer to lipid vesicles containing glycosphingolipids was studied. Both pertussis toxin and the B oligomer bound to lipid vesicles containing ganglioside GD1a. Binding of pertussis toxin to these vesicles decreased upon treatment of the vesicles with neuraminidase, suggesting that sialic acid residues are important for efficient binding of the toxin to GD1a. These findings are consistent with the idea that sialic acid residues may play a role in efficient pertussis toxin binding to glycolipids. Although the data reported here do not address whether GD1a acts as a functional receptor for PT on the surface of eukaryotic cells, these data provide information concerning the oligosaccharide structures to which PT is capable of binding. The localization of pertussis toxin within wild-type and mutant B. pertussis cells was also studied. Pertussis toxin in both wild-type cells and in mutants deficient in the secretion of the toxin fractionated with the membrane suggesting that pertussis toxin or its subunits may associate loosely with membranes or with proteins localized in the membrane.